ISTANBUL (Reuters) - Turkey has refused to allow Israel to take part in a NATO summit next month because the Jewish state has not apologized for the 2010 killing of Turkish activists in a raid on a ship taking aid to Palestinians, a Turkish official said on Monday.

Relations between the regional powers deteriorated sharply after Israeli commandos raided the Mavi Marmara aid vessel in May 2010 to enforce a naval blockade of the Gaza Strip and killed nine Turks in clashes with activists.

Last September, Turkey expelled Israel's envoy and froze military cooperation after a U.N. report on the raid failed to prompt an apology from Israel.

"We did not give our consent on that issue," a Turkish official told Reuters when asked if Turkey was blocking Israel's participation in a NATO summit in Chicago on May 20-21.